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paulengr

Better arc flash control

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This is slightly OT (as in it's not a direct control/PLC issue) but does anyone here have experience with any of the arc flash mitigation devices? The conventional techniques consist of essentially changing your trip devices (fuses and circuit breakers) to manipulate the trip curves. The latest thing is trying to sell arc resistant switch gear which essentially beefs up the enclosures and contains some sort of blast doors that allow the switch gear to vent in the event of an arc fault. Wonderful if you have everything buttoned up or are working in the low voltage compartment but utterly useless once you open things up (and they make opening it a much more difficult proposition). I'm talking about optical devices designed specifically to detect the light from an arc and trip the switch gear out regardless of whether it is open, closed, arc resistant or not. ABB sells two. there's the REA which is based on an unclad fiber optic cable. The other is the arc guard which is just traditional photoeyes. Both can be combined with a CT set up for high di/dt (if there's not a huge spike in current, it might be something other than an arc flash). The guy in the division at ABB that makes the photo-eye version said they've been selling these things for about 35 years, mostly to Europe. VAMP sells both technologies as well. Both connect to the shunt trip on the main bus circuit breaker for actually tripping. Both rely on very fast electronics to trip the breaker in a couple milliseconds. Effectively the only limiting factor in tripping is the length of time required for the circuit breaker to open. Square D's version is another variation, called the Arc Terminator. This one uses only the single photoeyes but includes something slightly different for the trip device. Instead of trying to open the main breaker immediately, it has a high speed contactor designed to short the bus to ground, theoretically providing a lower resistance path and extinguishing the arc, which is supposed to be faster than waiting for the main circuit breaker to open. Reason I'm asking is that I'm now working in a facility that uses medium voltage gear everywhere (lots of very large motors) and the arc flash rules are a real pain. According to plant rules, you have to suit up even to open/close a breaker manually unless you use one of those little remote trip boxes. They use breakers for EVERYTHING...there are no safety disconnects so the act of locking a motor out requires suiting up. On top of that, it is truly difficult trying to troubleshoot a simple starter with Class 2 gloves on while trying to peer through a tinted hood in a Darth Vader outfit. I just have zero experience with these photoelectric devices so I'm asking in case anyone else has had any dealings with them, good or bad. Edited by paulengr

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Have zero experience with these devices, but they sound interesting. I have a project coming up, adding a 1050ton chiller, new switch gear etc... Low voltage (480VAC ~ 1200 amps) Might make the down stream devices have a lower rating. Let me/us know what you find out.

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I have not used this equipment either but I feel your pain in the Vader suit. Good Luck, I will be interested to follow this thread to see how you make out.

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We occassionally see these things specd on boards in Oz. they are normally form 4 power boards (busbar). Form 4 has the explosion relieving vents (chimneys) if that helps and all sections are completely segregated from each other. The brand I have used before is Camto. The blurb says 'Laboratory tests have proved that the tripping pulse is generated in less than 1 millisecond after ignition of the arc. The arcing time is thus reduced to the operating time of the circuit breaker'. Also, 'Switchboard damage, both thermal and mechanical, is greatly reduced. Normally the installation can be operated again after cleaning and minor repairs'. I tend to agree as I have seen a board that 'went off' with these things installed. The biggest problem was cleaning up the mess in the busbar riser. There is usually not much room in there. The acrc was actually caused by a differential protection CT that we believe open circuited and started a fire. Could never prove it but that was where most of the initial damage was. Goes to show what an OC current CT can potentially do.

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My own followup so far: it appears that this stuff has been out in the field for quite a while and it definitely does work as advertised. I've gotten firm prices so far from ABB for their photo-eye version (not the fiber version). The trip units are around $3K-$4K and the photeyes are about $500 a piece give or take distances, options, etc. I'm hoping that the fiber stuff can cover a lot more cubicles for less money. I haven't done actual analysis yet (got to sit down with the data) but so far it looks like unless your switch gear is very slow (8-12 cycle opening times on the circuit breaker) you can get almost all the category 2-4 stuff down to 1 or occasionally to 0. More details forthcoming as I get everything together but sicne we don't have a copy of SKM or similar stuff, I'm having to run the equations by hand/spreadsheet and it's taking me a while to dig out all the data from GE's web site. The overriding "elephant in the room" is that no testing and/or provisions are made in the general case for what to do with the doors on the equipment being closed. A lot of companies are treating it just like shock protection (doors closed == safe), but that's obviously not the case. Arc blasts are enough to blow doors off and shoot flames, heat, etc., around the edges of the doors even if the doors hold. So some companies (mine included) and the all-but-worthless tables in NFPA 70E have taken the stance that doors opened/closed doesn't really matter. Some are differentiating between "switching activities" (as the NFPA does currently). It's a real quandry too because we all know that having the doors closed makes the situation safer but there's no test data corresponding to this situation, except in one case. The venting stuff is called "arc resistant". The cabinets are all built to something resembling explosion proof design criteria (which it almost what it is). The gear has to withstand the pressure wave long enough for the wave to start venting out the top/back of the gear. At least in the U.S. the test in question here is that they hang "T-shirt" type material 3.9 inches away from the face of the gear at all angles (simulating a person in cotton clothing standing in front of it) and purposely cause an arc fault. The low voltage compartments are also left open. The idea is that a person standing in front of it with the gear closed up and bolted shut gets nothing greater than a 1st degree burn (they are looking for no discernable damage to the cloth test strips). This is really nice so long as you are not dealing with any real or potential problems INSIDE the gear. It increases the cost of the gear by about 25%, and definitely increases troubleshooting time because now instead of just undoing latches, you have to undo a whole bunch of bolts to open the cabinetry. And once you start, all the nice arc resistant protection you paid for is now worthless.

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If a board has been type tested to form 4 vented enclosure standards the doors will not blow off. However, many companies will not order this style of board due to cost. The last busbar board I built and had tested for heat rise and kA rating was a 5000 amp bus, vented enclosures, 80kA and cost us about $90k Au - and that was over 10 years ago. Before doing this type of testing you really have to make sure you are going to use the certificate and that you can spread the cost of the excercise over a number of projects. One of the issues withy this today is that many suppliers are now supplying build-a-block cases that have been type tested. You have to go and do a course with them to get an assembly ticket but at the end of the day they work out cheaper. My main objection is that anyone can do the course and get the certificate without any knowledge or experience of how to design and build a switchboard. Also no knowledge/experience of the testing that goes on for type test certificates and the consequences of a 'mistake' either accidental or to cut costs - read cut corners. The 'real' switchboards builders are being forced to use these enclosures also due to cost - cannot compete with them.

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Arc flash hazards should be looked upon seriously and with adequate safety measures, training and knowledge. Technology is also instrumental in preventing arc flash hazards. Electrical accident preventions are always in placed but accidents still take a toll on the persons working on the premises. Electricity is a familiar and necessary part of everyday life, but electricity can kill or severely injure people and cause damage to property.

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